Certain aspects of the present disclosure generally relate to the field of marine geophysical processing and may have particular applicability to electromagnetic (EM) surveying.
In geophysical prospecting in a marine environment, cable configurations are often towed behind a vessel to deploy equipment such as energy sources and sensors. Energy sources may include EM field sources that generate EM fields that may be helpful in identifying geophysical features. Wires or cables for generating such signals may be bundled into source cables (also called “sources”) that are commonly jacketed or wrapped in an enclosure. Other geophysical energy sources may include air guns, marine vibrators, electric bipole antennae, and magnetic coils. The geophysical energy sources may be positioned at known locations in a geographic area. In a marine setting, the sources may be towed behind one or more boats traveling a prescribed course, usually in a group of aligned paths.
Measurements may be taken of wavefields that have been initiated by the geophysical energy sources, for example by sensors located in a sensor assembly such as a towed receiver streamer. The data set obtained typically contains information of interest indicating the geology of earth strata below the geophysical equipment.
EM surveying systems have used different methods to attach EM sensors (e.g., electrodes) to surveying equipment. For example, some systems embed EM sensors into a body of a sensor assembly cable segment or other cable assembly. Towed EM streamers (cables containing EM sensors and possibly other sensors, such as seismic sensors), can be used to measure EM fields. The measured EM fields can be generated by an EM field source (electrode), which can be a controlled source EM (CSEM). Variations in natural EM fields can also be measured. The EM field source is actuated at selected times, and measurements made by the various sensors on the streamer are detected and recorded for processing to determine marine subsurface properties, such as to locate subsurface resistivity anomalies in the rock formations below the water bottom and to quantify the content of materials, such as petroleum, that may be associated with such anomalies.
In some instances, the EM field source can be towed from a survey vessel that tows a number of streamers. EM fields generated by the EM field source and/or the survey vessel can affect conductive wires in the streamers that are between the survey vessel and the EM sensors (e.g., electrodes). The EM fields can generate disturbances (interference) that travel along the conductive wires, which can contaminate the EM field detections and/or measurements by the streamer's EM sensors. Unintended detection of the EM field may introduce noise into data obtained from the sensors. This may lead to less accurate data and additional processing in order to account for noise attributed to the unintentional detection.
Therefore, techniques and apparatus for geophysical surveying that reduce or eliminate unintended transmission of interference from the source's EM field are desirable.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized on other aspects without specific recitation.